Ambulatory medication error can be a lethal problem in children with chronic conditions; overdoses can be fatal and chronic under-doses (non-adherence) lead to serious injury. One in four children has a chronic condition and one in five takes a daily medication. While it is known that children with chronic conditions take only half of their medications, the amount of non-adherence which is caused by error has not been studied in any population. Ambulatory children are particularly vulnerable to preventable adverse drug events, due to the complexity of medication weight-based dosing, coordination between multiple caregivers, and the use of liquid medications. There is a critical need for generalizable, rigorous study of medication error and injury in children with chronic conditions to inform intervention. Pediatric cancer is an ideal model condition to understand chronic disease care: medication under- and overdoses are life-threatening, all forms of medication are nationally (intravenous, pills, liquids), and our preliminary data indicates that the prevalence of injuries due to medicatin error is very high (3.6% children). Our long term objective is to support the safe and effective use of medications among ambulatory children. In this proposal, among children with cancer, we aim to: 1) categorize rates and types of ambulatory medication error, including caregiver error, and preventable adverse drug events; 2) identify the contribution of error to non-adherence; and 3) identify individual, family, and healthcare system factors that contribute to ambulatory medication error. Our team is ideally suited to perform the proposed work, combining nationally known experts in ambulatory medication errors, including caregiver administration, adherence, and human factors. We will conduct a longitudinal cohort study of 300 children with leukemia and lymphoma from three centers. Families will participate in three home visits. Errors will be measured using a four-component approach we developed and validated: medical record review, medication bottles review, observation of administration, and caregiver interview. Non-adherence will be measured electronic monitoring devices and self-report. We will identify factors that contribute to error using an environmental scan and standardized online questionnaires completed by caregivers. The proposed work will substantially contribute to our knowledge of the epidemiology of error and the range of types of injuries due to medication error. In addition, we will identify factors contributing to ambulatory safety, including provider-family interaction, protective behaviors and tools, and the role of the health system in ambulatory safety. This will be the first study, to our knowledge, of the relationship of adherence and error. Knowledge gained will immediately benefit patients, families, and clinicians who can then take steps to minimize errors and associated injury. Researchers may build on this knowledge to apply human factor theory, widely used in the inpatient setting, to the ambulatory setting in children. The proposed work provides the critical information needed to develop and test novel interventions targeting ambulatory medication error and associated injury.